Cache system libraries

First of all, thanks a lot for putting this out there; I've started using it today and it worked flawlessly in the first try.

This is not an issue but a feature request. It seems like the cache is being stored at the end of ` Run mymindstorm/setup-emsdk@v5`, so system libraries built during the "normal" compilation are not cached.

In my test build, generating these libraries take ~1m 40s (out of ~2m). Extract:
```
Tue, 07 Jul 2020 12:11:04 GMT
cache:INFO: generating system library: libc.a... (this will be cached in "/home/runner/work/bq-tiler/bq-tiler/emsdk-cache/emsdk-master/upstream/emscripten/cache/wasm-lto/libc.a" for subsequent builds)
Tue, 07 Jul 2020 12:12:08 GMT
cache:INFO: - ok
Tue, 07 Jul 2020 12:12:08 GMT
cache:INFO: generating system library: libcompiler_rt.a... (this will be cached in "/home/runner/work/bq-tiler/bq-tiler/emsdk-cache/emsdk-master/upstream/emscripten/cache/wasm-lto/libcompiler_rt.a" for subsequent builds)
```

Is there any way to setup a post run step to the build (either manually in each job or by adding an option to the action) to get those system libraries to also be cached? As far as I know, these generated system libraries only depend on the emcc version, so caching everything together should be safe.